Liquid crystal display devices have been applied to, for example, watches, calculators, a variety of measuring equipment, panels used in automobiles, word processors, electronic notebooks, printers, computers, television sets, clocks, and advertising boards. Representative examples of types of liquid crystal display devices include a TN (twisted nematic) type, an STN (super twisted nematic) type, and a vertical alignment type and IPS (in-plane switching) type in which a TFT (thin film transistor) is used. Liquid crystal compositions used for such liquid crystal display devices need to satisfy the following requirements: being stable to external elements such as moisture, air, heat, and light; having a liquid crystal phase (nematic phase, smectic phase, and blue phase) in a broad temperature range mainly including room temperature as much as possible; having a low viscosity; and enabling a low driving voltage. Liquid crystal compositions need to have dielectric anisotropy (Δ∈) and refractive index anisotropy (Δn) optimum to individual display devices.
A liquid crystal composition having positive Δ∈ is used in horizontal alignment-type displays such as a TN type, an STN type, and an IPS type. In another type of driving that has been reported, molecules of a liquid crystal composition having positive Δ∈ are vertically aligned in a state in which voltage is not applied, and then a horizontal electric field is applied for performing display. A demand for a liquid crystal composition having positive Δ∈ has therefore further increased. In all types of driving, however, there have been demands for improvement of response speed, and a liquid crystal composition having a lower viscosity than typical liquid crystal compositions is needed to satisfy such demands. In order to develop the liquid crystal composition having a low viscosity, it is effective to decrease the viscosity of individual compounds contained in a liquid crystal composition. In the case where a liquid crystal composition is applied to, for example, display devices, the liquid crystal composition needs to exhibit a liquid crystal phase stable in a broad temperature range.
In general, in terms of production of a compound having a low viscosity, it has been believed that the compound preferably has a molecular framework in which multiple cyclic structures are directly bonded to each other via no linking group, namely a structure called directly connected rings. Compounds having positive Δ∈ and a structure in which three or more rings are directly connected to each other are generally highly crystalline; in the case where a liquid crystal composition containing such a compound is cooled, the crystals of this compound precipitate, which is problematic. A compound, for example, having the following fluorinated naphthalene structure (see Patent Literature 1)
has large Δ∈ and Δn and relatively low viscosity; however, the crystals thereof precipitate depending on the constitution of a liquid crystal composition. In order to enhance solubility, a variety of compounds into which linking groups are introduced have been studied. Although the introduction of linking groups increases viscosity to some extent, miscibility in a liquid crystal composition can be enhanced (see Patent Literatures 2 to 9). A compound having a —CH2O— group as a linking group is highly chemically stable, and a liquid crystal composition containing such a compound exhibits a liquid crystal phase stable in a broad temperature range; however, the viscosity of this liquid crystal composition is extraordinarily high, which is problematic.
In order to enhance the Δ∈ of a liquid crystal composition, a polar compound having large Δ∈ needs to be added to the liquid crystal composition in a high percentage. An increase in the amount of one polar compound to be contained, however, readily leads to generation of precipitates from the liquid crystal compound, and the upper limit of the amount in which the polar compound can be added is therefore determined. In a technique generally used for addressing such a problem, multiple compounds having a difference not in the basic skeleton but merely in the length of the alkyl chain are used to increase the polar compound content and to reduce precipitates. In this case, needless to say, analogous polar compounds having a difference only in the length of the alkyl side chain need to be produced at low costs, and an efficient synthetic route need to be determined. Hence, it is effective that the same synthetic intermediate be used and that a reaction route which enables a high yield be selected.